Making a planographic printing plate using silicone rubber or fluorine resin as the ink repellent layer without using dampening water, especially direct plate making which makes an offset printing plate without using any film for plate making has been used in the short run printing industry, and begins to be used also in the areas of offset printing and gravure printing because of such features as simplicity not requiring any high skill, speediness to allow a printing plate to be obtained in a short time, rationality to allow a system optimum in view of desired quality and cost to be selected among diverse systems. Especially recently in the rapid progress of output systems such as prepress systems, image setters and laser printers, new types of various planographic printing plates have been developed. The methods for making these planographic printing plates can be classified into methods of irradiating with a laser beam, methods of writing by a thermal head, methods of selectively applying voltages by pin electrodes, methods of forming an ink repellent layer or inking layer by ink jet, etc.
Among them, the methods of using a laser beam are more excellent than other methods in view of resolution and plate making speed.
For example, as directly imageable raw plate for waterless planographic printing plates, JP-B-42-21879, U.S. Pat. Nos. 4,519,40, 5,339,739 (6,243,1), 1,253,19, 5,928,3, etc. propose directly imageable raw plate for waterless planographic printing plates in which a heat sensitive layer containing an infrared absorbing material and a self oxidizing material and an ink repellent silicone rubber layer are laminated on a substrate. Furthermore, U.S. Pat. No. 2,470,14 proposes a directly imageable raw plate for waterless planographic printing plate in which a heat sensitive layer and an ink repellent silicone rubber layer are laminated on a substrate. However, in these directly imageable raw plate for waterless planographic printing plates, since the heat sensitive layer is hard and fragile, the stress acting on the plate surface during offset printing acts intensively at the interface between the heat sensitive layer and the silicone rubber layer, to cause adhesion rupture. Furthermore, the heat sensitive layer is likely to be damaged, and according to the increase of printed sheets, the heat sensitive layer below the ink repellent layer is damaged in the non-image area, and this phenomenon erodes the ink repellent layer, to lower image reproducibility disadvantageously. As a result, the printing durability of the printing plate becomes insufficient disadvantageously. Studies have been made for the purpose of improving the printing durability. U.S. Pat. No. 2,470,16 proposes a plate in which a silicone rubber layer is anchored by an adhesion accelerator such as a silane coupling agent, and according to this proposal, though the adhesiveness to the heat sensitive layer is improved, practically sufficient printing durability cannot be obtained. Thickening the ink repellent layer has also been attempted, but the decline of sensitivity caused by thickening and the shortening of ink mileage occur disadvantageously. To overcome these problems, various studies have been made for photosensitive waterless planographic printing plates. JP-A-1-161242, JP-A-1-154159, etc. propose to thicken the ink repellent silicone rubber layer, while compensating the shortening of ink mileage due to thickening, by adjusting the cell depth, for example, by embedding an ink acceptable material. In this case, the problem of decline of sensitivity remains unsolved, and an additional new step of embedding an ink acceptable material, etc. poses another problem of practical inconvenience. A plate with a filler added into the ink repellent silicone rubber layer is also studied, but it is insufficient in the improvement of printing durability though the resistance against the flaws caused by the washing of plate surface, etc. can be improved. In addition, there arises a problem that the ink repellency required in the silicone rubber layer declines greatly. U.S. Pat. No. 5,379,698 describes a directly imageable raw plate for waterless planographic printing plate using a thin metallic film as the heat sensitive layer. In this case, since the thin metallic film itself allows the transmittance of the laser beam to some extent, the sensitivity declines. To prevent it, a reflection layer must be formed below the thin metallic layer, to require an additional coating step disadvantageously in view of cost. JP-B-6-199064, U.S. Pat. No. 5,353,705 and EPO 580393 also describe directly imageable raw plate for waterless planographic printing plates using a laser beam as the light source. The original printing plates of heat destruction type use carbon black as a laser beam absorbing compound and nitrocellulose as a thermally decomposing compound. These printing plates are better than the printing plate using a thin metallic film in laser beam absorption efficiency, but have a problem that they are likely to be flawed during printing and low in printing durability since the adhesive strength between the silicone rubber layer on the surface and the heat sensitive layer is weak. Furthermore, though carbon black is used as a laser beam absorbing material, all the primary grains of the carbon black used in the above patent are 30 .mu.m or more in diameter, and it cannot be said that the carbon block absorbs the light of a semiconductor laser (about 800 nm in wavelength) efficiently. The reason is that the optical density as a printing plate which is one of indicators of laser beam absorption efficiency does not become maximum at the grain size. The optical density becomes maximum when the grain size is about 20 .mu.m, and the blackness declines at a grain size of larger than 30 .mu.m. If the grain size is smaller than 15 .mu.m, dispersibility declines. Furthermore, since the carbon black stated in said patent is large in oil absorption, i.e., has a high structure, it has a problem that the solution destined to be the heat sensitive layer cannot be applied to form a uniform film since carbon black grains cohere to each other, to raise the viscosity of the solution. On the other hand, the directly imageable raw plate for waterless planographic printing plate with a thin metallic film as the heat sensitive layer has a problem that a reflection layer must be formed below the thin metallic film since the thin metallic film allows some transmittance of the laser beam, though a very sharp image and high resolution can be obtained since the heat sensitive layer is very thin. Moreover, few apparatuses are introduced for efficiently and stably mass-producing these directly imageable raw plate for waterless planographic printing plates.
The present invention has been created to improve the respective disadvantages of the prior arts, and provides a directly imageable raw plate for waterless planographic printing plate remarkably improved in printing durability without lowering the developability, image reproducibility, printability and solvent resistance of the plate by using specific compounds or materials for forming the heat sensitive layer and the heat insulating layer as flexible layers, and specifying the initial elastic modulus and 5% stress as tensile properties for the flexibility of the heat sensitive layer or the heat insulating layer or a laminate consisting of both the layers.